In time-of-flight mass spectrometry, ions produced from a sample are accelerated by an electric field along a flight path in a pulsed fashion. The field provides each ion with the same kinetic energy. Since kinetic energy is related to mass and velocity by K=½ mv2, the velocity of a given ion, and therefore its arrival time at the detector (positioned at the far end of the flight tube), depends on its mass to charge ratio m/z according to the following relationship.
  t  =      d    ⁢                  m                  2          ⁢          zV                    where d is the length of the flight tube and V is the acceleration potential. The mass resolution of a time-of-flight mass spectrometer is therefore directly determined by the time resolution of the detection system. An alternative method for generation of sample ions involves using a neutral sample within a DC field and to effect ionization using a laser (or other means). The sample molecules do not ‘see’ the extraction field until they are ionized. Microchannel plates (MCPs) are usually used to detect the ions. MCPs are thin glass plates laser-drilled with an array of holes. The plates are resistively coated, such that an ion striking the front of a channel elicits the emission of electrons from the surface. When an appropriate potential difference is applied across the plate, the electrons are accelerated through the channel, producing more electrons on every collision with the channel surface. For each ion striking the front of a channel, up to 103 electrons are emitted from the back face. Typically, two or three MCPs are stacked together to increase the gain to 106 or higher. In most time-of-flight experiments (and in all commercial mass spectrometers), the total electron current produced by the MCPs is measured.
Currently employed detection techniques are limited in their time (and therefore mass) resolution primarily by the time resolution of the readout electrons used to read a time-dependent signal from the MCP's.
We seek to provide an improved charged particle spectrum analysis apparatus, and in particular an improved time-of-flight mass spectrometer.